Bioremediation is the use of living organisms for detoxication of hazardous wastes. It can involve either the introduction of specific organisms and/or the stimulation of indigenous bacteria. The technique was first experimented in the 1970's to treat contaminated soils and aquifers. However, hurdles in the regulatory processes and constant pressure from environmentalist groups had reduced the use of bioremediation to marginal levels.
In recent years however, several advantages have been recognized for bioremediation, particularly in situ bioremediation. Among others, it has been found that the technology is specific, relatively cheap and quicker to use than most other remediation techniques. The technique can be used to degrade a wide variety of organic compounds present at various levels in contaminated soils. One of the most important advantages of biological degradation over conventional techniques is the fact that the contaminants are usually broken down to harmless substances whereas conventional techniques usually only temporary displace the problem or transfer the contaminants to another medium.
Various approaches to bioremediation of soils have been tested and compared. One preliminary conclusion that seems to be drawn from those tests is that microorganisms occurring in nature are so versatile and so adaptable that most applications of bioremediation could rely almost exclusively on the use of natural organisms that have not been modified in any way, thereby avoiding, at least for now, the introduction in natural systems of genetically engineered organisms.
The most common technique used in bioremediation involves the stimulation of naturally occurring bacteria residing at the site of contamination. It is usually called "biostimulation". By addition of the appropriate nutrients, principally oxygen, phosphorus and nitrogen, and by maintaining optimum growth conditions, it is possible in some instances to favour increased multiplication of the communities of indigenous organisms that together are capable of degrading undesirable contaminants. Hence, the addition of microorganisms to the site is not required in biostimulation. A common variation of the "biostimulation" technique is land-farming. In land-farming, chemical nutrients are added to soil, often in an excavated pile, while adequate oxygenation is assured by frequent turning or dishing of the soil.
Implementing biostimulation usually requires a certain amount of laboratory testing, to ensure that there are sufficient numbers of indigenous microbes on site, and to determine the optimum conditions to enhance their growth and biodegradative action. This preliminary characterization usually leads to treatability studies to establish that the site can be remediated economically. In situ bioremediation usually requires extensive engineering to introduce nutrients to the site, through injection wells, infiltration galleries and the like. Land-farming, in contrast, can be accomplished simply by spraying the nutrients onto soil piles that are frequently mixed and aerated.
In U.S. Pat. No. 4,849,360, Norris et al. describe a process using biostimulation for confining contaminated soils and degrading the hydrocarbons they contain. The method comprises the use of indigenous microorganisms, nutrients, water and a suitable gas distribution system. The amount of contaminated soils to be treated is adjusted by evaluating the capacity of the gas distribution system to create optimal aerobic conditions (see column 2, lines 47-50). The method also involves evaluating the native microbial community in the soil and creating proper conditions for this community to grow as much as possible (see column 4, lines 12-45). One of the major drawbacks of this method appears to be the fact that the soils to be decontaminated must be confined in a container.
Another example of a decontamination system using biostimulation is described in the March 1991 issue of Chemical Engineering in an article entitled "Mighty Microbes". For this system, liquids are sprayed on the contaminated soil pile, and air may be blown or suctioned through the mass by a system of pipes located under the mass. This is described as a "wet" technique for the on-site processing of soil. Contrary to the technique described in U.S. Pat. No. 4,849,360, soil is not confined to a container. The sprinkling system is used to add water and nutrients and the air distribution system buried in the pile increases oxygen supply. Although the method has proven to be quite interesting for the degradation of some contaminants, it has been found to be relatively time consuming and somewhat limited for degrading more chemically stable contaminants such as PCB's. As shown at page 33 of the article, decontamination of a contaminated soil containing PCP and creosote led to a 58% reduction in contaminants in 3 months. Generally, this is not sufficient to meet the levels required by the regulatory authorities in North America.
An alternative to biostimulation is "bioaugmentation". This technique consists in introducing non-native cultures, previously selected from other sites for their ability to degrade specific wastes. These microbial products are usually blends of different species of strains. In recent years, companies have begun selling microbial blends purported to be active against hazardous compounds, including use for in situ waste remediation. Most common are products for degradation of hydrocarbons and petroleum distillates, but several manufacturers also sell microbial products with claimed activity against aromatic compounds and other hazardous chemicals. In addition to these commercially available cultures, there are several microbial isolates that have shown success in the laboratory in degrading hazardous wastes, such as the white-rot fungus, which can degrade lignins and many other aromatic compounds.
Waste treatment technologies based on the principle of bioaugmentation have also been developed. In U.S. Pat. No. 4,850,745, Hater et al. describe a bioaugmentation technique by which a system for treating soil contaminated by petroleum hydrocarbons is designed by applying in a dry form a suitable bacterial culture capable of degrading petroleum hydrocarbons to the bottom of an excavated cavity. A system of distribution piping capable of supplying nutrients directly to the cultures and also an air flow through the area containing the cultures is provided to maintain optimal growth conditions. The system described by Hater et al. seems to be operated in a closed circuit. In other words, Hater et al. do not teach or suggest the subsequent introduction of microorganisms once the initial inoculation has been made.
In U.S. Pat. No. 4,952,315, another type of bioaugmentation technique is described. Saab discloses a process for eliminating hydrocarbons contained in a contaminated soil. The desired result is obtained by using a microbiological treatment involving the use of emulsifiers permitting the separation of the contaminants from the soil in which they are found. The contaminants can then be degraded by using a biological process requiring endogenous bacteria. This approach can be somewhat lengthy as it is required to bring the contaminants in a fluid phase before having the possibility of degrading them through the action of microorganisms.
One of the most promising applications for bioaugmentation appears to be in the degradation of oil spills, since the biology of hydrocarbon degradation has been well studied. Unfortunately, most of the methods that were used so far to decontaminate major oil spills such as the Mega Borg and Exxon Valdez spills could not generate conclusive data. Furthermore, although bioaugmentation allows the introduction of microbes tailored for a given waste, it has difficulty working in practice because competition from natural microbial populations requires large inoculum sizes, and because cultured organisms cannot always handle the stresses present in natural environments.
Bioremediation offers some concrete advantages over competing methods. It is a destructive technology that offers a permanent solution to hazardous waste problems, without the need to remove the wastes off-site. It utilizes a natural process that does not itself create environmental problems. Even though in situ biostimulation requires preliminary laboratory assessment, it can be implemented quickly and inexpensively at most sites. Soil bioremediation has been estimated to be far less expensive than incineration or land disposal, and competes well with other available options, like recycling.
However, bioremediation, either through biostimulation or bioaugmentation, has its limitations. Hence, bioremediating soil will generally take longer than excavation for incineration or landfilling. Also, biostimulation is often insufficient to provide degradation of contaminants at acceptable levels while in the case of bioaugmentation, one of the major problems seems to reside in the fact that it is difficult to maintain the cultures introduced at the beginning of the process to optimal levels.